Recently, attempts have been made to form a field-effect transistor (hereinafter, it may be called simply FET in this specification) using a metallic oxide film for a channel layer. Especially, thin film transistors, using ZnO, InGaZnO, and ZnSnO, etc. for a channel layer (hereinafter, it may be called simply TFT in this specification) have been developed. However, the main motive thereof is that the manufacturing process being performed at low temperatures and being simple. Research and development have been actively pursued for forming TFT directly over a so-called flexible film such as a plastic film, by taking the advantage of this low temperature and the feature of simplicity.
Recently, the TFT most widely used is one using an amorphous or polycrystalline silicon for the channel layer. However, there is a problem that a high temperature process is necessary for manufacturing devices of a silicon TFT and is unsuitable for manufacturing over a flexible substrate.
In an oxide TFT, a source electrode and a drain electrode are formed by having a conductive film made of a material such as a metal and ITO (Indium Tin Oxide), which are different from the channel layer, contact with the channel layer. Therefore, the contact resistance between the metallic oxide film which is a semiconductor and the conductive film, and the parasitic resistance between the channel region and the source electrode or the drain electrode become a reason for preventing an improvement in the TFT characteristics. Moreover, when the contact resistance and the parasitic resistance are high, the variations thereof becomes a big factor of the variations in the TFT characteristics and it is a reason for preventing performance improvement when a circuit and display device, etc. are manufactured. Especially, in an organic LED (Organic Light Emitting Diode: OLED) display device using the TFT in an analog manner, variations in the TFT makes the image quality deteriorate.
For these reasons, in the oxide TFT, it is required to decrease the contact resistance and the parasitic resistance and to decrease the variations thereof.
In order to solve the above-mentioned problems, JP-A-2007-073699 and JP-A-2007-250983 disclose a means for decreasing the contact resistance and the distribution thereof by increasing the conductivity of the desired region in the metallic oxide film and forming the source region and the drain region. It is also disclosed that the parasitic resistance and the variations thereof are decreased by forming the source region and the drain region self-aligned.
It is known that the conductivity of the metallic oxide film can be controlled by the oxygen content and the impurity hydrogen content in the film, and it is used in the above-mentioned JP-A-2007-073699 and JP-A-2007-250983. In the metallic oxide film, since the oxygen defects or impurity hydrogen work as n-type donors and carrier electrons are created in the film, the conductivity becomes higher and the film resistance lower as the oxygen content decreases or the hydrogen content increases.